Distinct Valley Polarization in Vertical Heterobilayers: Difference between Edge- and Center-Nucleated WS2/MoS2.

ABSTRACT

Structural imperfections can cause both beneficial and detrimental consequences on the excitonic characteristics of transition metal dichalcogenides (TMDs). Regarding valley selection, structural defects typically promote valley depolarization in monolayer TMDs, but defect healing via an additional growth process can restore valley polarization in vertical heterobilayers (VHs). In this study, we analyzed the valley polarization of center-nucleated and edge-nucleated VHs (WS2/MoS2) grown using a controlled growth process and discovered that defect-related photoluminescence (PL) is strongly suppressed in the center-nucleated VHs due to defect healing. Additionally, we demonstrated that the valley polarization of lower-lying intralayer excitons is more sensitive to the defect density of the sample than to higher-lying intralayer excitons. Despite defect healing in the center-nucleated VHs, the temperature-dependent PL study indicated that valley depolarization of the lower-lying intralayer excitons becomes significant below 100 K because of stronger hybridization of defect states. Also, we conducted a comprehensive study on the excitation intensity dependence to investigate the electron-doping-induced Auger recombination mechanism, which also contributes to valley depolarization of intralayer excitons via regeneration of intervalley trions. Our findings provide valuable insight into the development of VH-based valleytronic devices.